From 2GreenEnergy Mega-Contributor Gary Tulie: Do We Need a Coral Ark?
(Note: I’m always happy to see my colleague Gary Tulie involve himself in our discussions, and I hope readers will find his words here on the subject of coral reefs to be valuable. – Ed.)
In this recent post, we talked about the threat posed to coral reefs by climate change, and as was rightly pointed out by frequent 2GreenEnergy commenter Glenn Doty, the associated risk posed by ocean acidification.
Coral is a vitally important and hugely biodiverse habitat with great value to humanity as a source of food, protection from storm surges, sheltered anchorage, and the inherent value of a beautiful and diverse ecosystem.
Coral reefs face a number of threats, all of which one way or another need addressing. (Not necessarily in the following order).
Warming Oceans. As oceans warm, especially where the process is rapid, the saprophytic algae which grow in corals giving them much of their colour and beauty become stressed, resulting in coral bleaching. When this goes on too long the health of the reef can be seriously affected.
Ocean Acidification. Corals use a huge proportion of their metabolic energy extracting carbonates, primarily calcium carbonate, from sea water to form coral skeletons. Calcium carbonate becomes more soluble the more acid it is exposed to, so that even a very small degree of acidification makes it significantly more difficult for corals to build.
Unsustainable Harvesting and Reef Activity. This varies from cyanide, electric, and dynamite fishing to damage from trawling, inappropriate anchoring, dredging for sand, and excessive collection of corals for aquariums, tourist souvenirs, etc.
Damage from Land-based Activities. Activities many miles away can cause damage to reefs. Such distant threats include the following.
• Excess runoff of fertilizer can result in excessive growth of seaweed followed by eutrophication as biological oxygen demand exceeds supply. This problem is made worse by warming seas as oxygen solubility in water decreases with increasing temperature.
• Soil erosion can result in excessive silt covering and choking corals. This is also a threat around dredging and construction activities.
The Need for Mitigation
Around 40% of the world’s coral reefs have died over the last 30 years, and the threat from warming and acidifying oceans continues to grow due to ongoing and increasing CO2 emissions.
Without help, the prospects for healthy reefs in 2100 look seriously threatened.
If we wish to see healthy coral reefs surviving and flourishing even as the above threats remain in place, it is clear that mitigating action is required, but how do we mitigate with sufficient effectiveness at sufficient scale?
Even with massive resources, it is difficult to imagine arresting the loss of coral at today’s levels, so a more strategic “Coral Ark” approach might be necessary to ensure survival of the most vital and diverse reef environments, and to do so in a wide range of geological areas.
Having selected a range of reef habitats for intensive protection and support, it will be necessary to maintain this support long term, possibly over hundreds of years.
Mitigation Strategies
How can we create a “Coral Ark,” and what might it look like and consist of?
Electric Reefs
The Coral Reef Alliance has built a number of “electric reefs” consisting of structures made of steel reinforcing bars sunk onto the sea bed onto which pieces of coral are tied. A small DC current is passed through the structure resulting in precipitation of calcium carbonate onto the steel structure, and a major reduction in the biological energy expended by the attached coral for the construction of its structure.
Such reefs greatly speed the formation of coral and the growth of crustaceans on the reef, and so far have proved resilient to bleaching events on adjacent reefs during periods where the ocean is unusually warm.
Whilst highly effective as a coral nursery, such structures are labour intensive, have significant costs, and are unlikely to be adopted at large scale, especially in the poorer regions of the world.
Seaweed Farming
One way to reduce the threat of fertilizer runoff is to cultivate and harvest seaweed and other useful aquatic plants such as bullrush in rivers and estuaries adjacent to coral reefs—where necessary aerating the water to replace any loss of oxygen in the water.
OTEC
Fortuitously, the majority of coral reefs occur in regions which are also suitable for Ocean Thermal Energy Conversion.
As a byproduct of electricity generation using this method, large quantities of cold water are brought up from the ocean depths and discharged at the ocean surface. In some regions, it should be possible to discharge in a controlled way cooling the waters which flow over nearby reefs thereby cooling the reef and protecting it from bleaching events.
Assuming that water from the ocean depths of 20 centigrade cooler than at the surface, for every thousand litres of water brought up from the ocean depths and discharged at the surface, around 23 kWh of surface water cooling will be delivered.
It takes very little energy to lift water from the deep ocean to the surface.
Assuming a pipe with 100 square metre cross section, and a vertical flow rate of 0.5m/s it is theoretically possible to pump 50,000 litres per second to the surface by using 12.5 kW of pumping energy.
Such an arrangement would deliver a massive 3.6 GW of surface water cooling. This would represent a massive coefficient of performance of 288,000 for the cooling of surface waters (even disregarding the fact that once moving, saline gradient will tend to make the rising water more buoyant than that at the surface).
Such cooling could be used even in the absence of an OTEC device with just a simple pump similar to an underwater propeller in a tube oriented to drive water to the surface. This would work in shallower water areas with a lower temperature difference between the surface and deep-ocean as well as in areas where the deep ocean is nearby.
Such a strategy would overcome the thermal stress on reefs by delivering cooler water, and dilute the more acidic surface waters thereby allowing the coral a better chance of surviving acidification. The cooler water would also be able to hold more oxygen in solution, and carry a wide range of minerals promoting productivity.
